1. Field of the Invention
The present invention relates to an antenna and, specifically, to an antenna including a radiator made smaller than a conventional radiator.
2. Description of the Background Art
A general antenna includes a radiator as a device for transmitting and receiving radio waves. By way of example, a Yagi antenna generally used for receiving television broadcast signals is formed of a director, a radiator and a reflector.
Conventionally, various and many techniques related to antennas have been disclosed. For example, Japanese Patent Laying-Open No. 49-040651 discloses a jig, which has holes for forming conductive patterns corresponding to antenna shapes by applying conductive coating, for mass-producing various antennas including conical antenna and Yagi antenna in a simple manner.
Antenna types vary widely, and antennas have various names reflecting operation principle, characteristics or shape. One type of such antennas is “fan-shaped dipole antenna.” The fan-shaped dipole antenna is characterized by its wide range of operable frequency.
FIG. 18 shows an example of the fan-shaped dipole antenna.
Referring to FIG. 18, a radiator 103 includes dipole elements 110 and 112, which are plate-shaped conductors. Dipole elements 110 and 112 are provided in symmetry about a Y-axis, and respectively connected to a power feed line (such as a feeder or a coaxial cable) at power feed points 114 and 116. Each of the dipole elements 110 and 112 has a trapezoidal or triangular shape, having its width along the Y-axis direction made wider further away from the power feed point.
The dimensions in the X-axis direction and Y-axis direction of radiator 103 are 210 mm and 76 mm, respectively. Generally, frequency range of radio wave that can be received by an antenna depends on the length and width of the radiator. Radiator 103 is used for receiving radio wave of UHF (Ultra High Frequency) television broadcast.
FIG. 19 is a graph representing a characteristic of radiator 103 shown in FIG. 18.
Referring to FIG. 19, the abscissa of the graph represents frequency, and the ordinate represents VSWR (Voltage Standing Wave Ratio).
In FIG. 19, the frequency range is 470 MHz to 806 MHz, which range covers both UHF television broadcast frequency ranges of Japan and the United States. In Japan, frequency range of broadcast radio wave of UHF television broadcast is 470 to 770 MHz (13 to 62 channels). Particularly, frequency range of digital terrestrial broadcast is 470 to 710 MHz (13 to 52 channels). In the United States, frequency range of broadcast radio wave of UHF television broadcast is 470 to 806 MHz.
In FIG. 19, a curve G100 represents variation of gain with respect to the frequency, while a curve V100 represents variation of VSWR with respect to the frequency. The gain becomes higher as the frequency is higher, and peaks around 761 MHz. On the other hand, VSWR lowers as the frequency becomes higher. The frequency at which the gain attains as high as possible and VSWR attains as low as possible corresponds to the peak antenna characteristic. In the example shown in FIG. 19, the antenna characteristic peaks at a frequency near 761 MHz.
FIG. 20 is a graph representing another characteristic of radiator 103 shown in FIG. 18.
Referring to FIG. 20, the abscissa of the graph represents frequency, and the ordinate represents half width (indicated by H.P.A (H.P.A is an abbreviation of ‘Half Power Angle’.) in the graph) and front-to-back ratio (indicated by F/B in the graph). The half width is an angular width at which the radiation intensity (radiation power) attains one-half (½) the maximum value. The front-to-back ratio is the ratio of radiation intensity in the direction of a reference point (angle 0°) to radiation intensity in the direction in the range of 180°±90° from the direction of the reference point. It is noted that directivity of the antenna transmitting radio waves is the same as the directivity of the antenna receiving the radio waves.
A curve H100 represents variation in the half-width with respect to the frequency, and a curve F100 represents variation in the front-to-back ratio with respect to the frequency. As can be seen from curve H100, the half-width becomes smaller as the frequency is higher (beam width becomes narrower). In contrast, the front-to-back ratio is kept around 0 dB regardless of the variation in frequency, as indicated by curve F100.
In FIG. 19, the frequency at which antenna characteristic peaks is around 761 MHz and considerably different from the center (around 653 MHz) of the frequency range. From the practical viewpoint, when the characteristic peak is to be set near the center of frequency range, the length of radiator 103 in the X-axis direction must be made longer than 210 mm.
When an antenna is installed outside, a longer radiator poses no problem as there is sufficient space. An indoor antenna, however, has restrictions in installation space and position. Therefore, an indoor antenna must be as small as possible, and hence, a radiator for an indoor antenna should preferably be as small as possible.
A small radiator may be used both for an outdoor antenna and an indoor antenna. The conventional radiator, however, unavoidably becomes large when better characteristics are to be realized, and reduction in size has been difficult.